In recent years, the trend of manufacturers to fabricate products using modular subsystems has created a need for small, light-weight, devices that are easy to incorporate in larger systems.
Generally, monochromators and spectrographs manufactured for incorporation as subsystems in larger instruments take the form of rectangular metal boxes with inlet slits and provision for, in the case of a monochromator, an outlet slit or, in the case of a spectrograph, a detector array. The inputs and outputs of these devices are typically placed at the ends of their optical paths. While both the inlet and the outlet of a device may be on the same side, they are usually placed on adjacent sides of the box containing the spectrograph.
In one class of spectrum analyzer, the device functions by inputting light through the input slit, causing it to fall upon a concave focusing diffraction grating. The concave focusing diffraction grating then causes the light to be reflected at different angles and to be focused at different positions dependant upon its wavelength. The light at one or more wavelengths, which is to be monitored is then collected by a detector device at the relevant position focus. The detector device generates a signal, either in real time or over extended periods, relating to the intensity of the light falling on it.
An advantage of a modular approach to optical subsystem manufacture is that it facilitates precise location and orientation of optical components in specialized engineering and manufacturing environments. For such applications, compactness is highly desirable.
One possible approach for a compact spectrograph system is the use of a so-called Littrow mounting. In the Littrow mounting, the input and output paths of the monochromator or spectrograph are both contained in a plane parallel to the grooves of the grating substantially perpendicular to the surface of the grating. The input and output paths are also configured to be substantially coincident with the central axis of the grating at a relatively small angle with respect to each other, so that the same optics may be used in both the collimation of light falling on the grating and the focusing of light leaving the grating. The use of the same optics in both the input and output paths has the advantage of substantially cancelling a significant portion of the aberrations associated with the system. In addition, there is an economy of space.
One approach for a compact modular analyzer is illustrated in U.S. Pat. No. 4,850,706 of Mikes. Here a concave focusing holographic diffraction grating is operated in near-Littrow configuration. In this device, the input light and an output spectrum are substantially in the same plane, a plane perpendicular to the grating and parallel to the grooves of the grating, and depart from the central axis by less than ten degrees. Compactness is achieved by folding the input and output light paths using a pair of mirrors which are substantially at right angles to each other and define an apex that faces the grating surface. Consequently, the light path extends from between opposite sides of the spectrum analyzer module. This is in contrast to the configuration of prior systems which evolved around perpendicular input and output paths. A drawback of a Littrow mounting is that it severely limits the grating designer's freedom to optimize system characteristics.
Another approach to a compact high quality spectrum analyzer module is disclosed in my earlier co-pending patent application, Ser. No. 07/960,091, U.S. Pat. No. 5,371,586 entitled Low Aberration Diffraction Grating System, filed Oct. 9, 1992. In this system, collimated light is input to the spectrograph and analyzed light is focused on a wall of the modular spectrum analyzer adjacent to the wall in which input light enters. The absence of collimating optics and the use of relatively customary grating geometry gives this system exceptional power, although there is a small compromise to be made in the area of system compactness.